Fly ash is a finely divided residue that results from the combustion of pulverized coal. For example, fly ash is a by-product of coal combustion in the generation of electrical power by coal-fixed power stations. Fly ash is collected in particulate control systems which remove the ash from the flue gases rising from the combustion chamber of a power station boiler. The ash is then transferred to a storage silo, sluiced to an ash pond or landfilled.
Fly ashes are fine powders generally comprised of spherical or rounded glassy particles. Chemically, fly ash particles are primarily comprised of variable silica, alumina, and iron oxide content. The percentages of these chemicals found in the residue vary depending on the type of coal burned. Angular particles found in fly ash compositions may include other mineral particles, unburned coal or char. Furthermore, fly ash is a pozzolan, accordingly, it reacts with calcium hydroxide to form additional cementitious compounds in binders, such as Portland cement.
There are two primary classes for fly ash recognized by the American Society for Testing Materials (ASTM). Class C fly ash is the residue resulting from the combustion of lignite or sub-bituminous coal. Class C fly ashes are recognized as having some cementitious properties, as well as pozzolanic properties, due to the presence of appreciable calcium oxide (CaO) in their chemical composition. Class F fly ash is the residue resulting from the combustion of bituminous or anthracite coal. While Class F fly ash is a pozzolan, in itself it has no cementitious properties.
Annual production of fly ash is estimated to be 80 million tons, making it one of the most abundant industrial by-products in the world. At the present time only about 6 to 10 percent of this material is consumed through utilization, the balance is being land filled. Accordingly, fly ash utilization is a benefit to the environment, as it lessens the strain on ever decreasing available landfill space.
Air-entraining agents produce minute air bubbles on the order of 1.0 to 0.1 millimeters in diameter, that are uniformly distributed throughout a mixture of concrete and closed off from outside air so as to act as a kind of additional "fine aggregate". In general, air-entraining agents are made either from wood resins, vegetable or animal fats or oils, or the fatty acids or soaps of the latter, or from wetting agents or synthetic detergents.
In the prior art, it has been widespread practice to entrain air in Portland cement. Portland cement is a hydraulic cement produced by pulverizing Portland cement clinker usually containing calcium sulfate. The entrained air affects the properties of both fresh and hardened concrete produced using the Portland cement. In this respect, in properly proportioned fresh concrete, at equal slump, air-entrained concrete is considerably more workable and cohesive than similar non-air-entrained concrete, except at higher cement contents. Segregation and bleeding of "ordinary" Portland cement (OPC) mixtures (i.e., Portland cement mixtures without fly ash) are also reduced by the entrainment of air. As to hardened concrete, extensive laboratory testing and long-term field experience has demonstrated that properly air-entrained concrete can better resist the action of freezing and thawing.
The properties of fresh and hardened concretes comprised of Portland cement (PC) mixtures (i.e., Portland cement mixtures containing fly ash as either a mineral admixture or a direct cement replacement) and ordinary Portland cement (OPC) mixtures (i.e., Portland cement mixtures not containing fly ash), are similarly affected by entrained air. However, the presence of the fly ash in Portland cement (PC) mixtures may affect the air content in fresh concrete, as well as the stability of the entrained-air voids. Accordingly, use of fly ash in air-entrained concrete will generally require modifications to the dosage rate of the air-entraining admixture. In this respect, to maintain a constant air content, air-entraining admixture dosages must usually be modified depending on the carbon content, LOI, fineness, and the amount of organic material in the fly ash. For instance, fly ashes with LOI values equal to or greater than 3 percent will generally require a significant increase in the dosage of the air-entraining admixture. In contrast, fly ashes with LOI values less than 3 percent will generally require no appreciable increase in the air-entraining admixture dosage. It should be noted that fly ashes with an LOI of 6 percent or higher, as determined in accordance with ASTM C-618, are defined as high LOI or high carbon fly ashes (ASTM C-618 is a specification established for the use of fly ash, as a mineral admixture in Portland cement (PC) concrete). Class F fly ash, for instance, may have an LOI of as high as 30%. Factors affecting the LOI value in Class F fly ash include the type of coal burned, moisture content of the fuel, type of burner, combustion temperature, boiler demand, weather conditions, type of ash collection system, pollution control devices and techniques.
A typical air-entraining admixture is neutralized vinsol resin. Tests have shown that air-entraining admixtures comprised of neutralized vinsol resin do not perform well in cementitious mixtures having fly ashes with high LOI values. In this respect, carbon particles or char (i.e., organic unburned coal remnants) in the fly ash are porous and can have very high specific surface areas similar to that of charcoal. All forms of charcoal are porous and may be used to absorb gases and/or purify or clarify liquid. Accordingly, in Portland cement (PC) concrete containing high LOI fly ash, the carbon particles tend to absorb the air-entraining admixture liquor or liquid film (i.e., the lamellae) surrounding the air dispersion in the mixture. This action allows the air globule to escape. The problem of admixture absorption by carbon particles is further increased, if for example, the concrete mixture is subjected to prolonged mixing periods, or if the concrete mixture is subjected to pressure such as would be experienced within a pump line. The foregoing conditions can further complicate predicting an air content per dosage of air-entraining admixture in Portland cement (PC) concrete mixtures having high LOI or high carbon fly ash.